So says Holden to Shepard in a clarifying moment during the ninth episode of Mindhunter, as the boss is downstairs grilling the four members of the BSU about a missing section of the Richard Speck interview transcript. The discussion gets down to some basic questions that have haunted the nascent unit all season: What’s appropriate? What’s scientific? What questions will elicit useful responses? And will they get what they need entirely from coloring inside the lines? Now that they have major funding and an audience outside their basement expecting results, they cannot continue to function as a rogue unit hell-bent on dismantling the square protocols of the FBI. They need to be a proper operation.

At the same time, the divisions within this circle of five are telling. Shepard is obviously an old-school, by-the-book guy, but he gets some support from Carr, who comes from the world of academia, where scientific studies are expected to abide by rigid methodologies. She doesn’t see someone like Speck as some golden goose, but one source of data among many to be aggregated alongside 40 other subjects yet to be interviewed. When she first met Bill and Holden, she talked about prisons as the perfect laboratory setting for conducting this sort of study, but any improvisation on their part contaminates the lab. It becomes harder for her to sort out which information is useful and which information has been tainted by role-play or provocation.

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Yet Carr is not in the room for these interviews. Nor is Shepard. Nor is Gregg Smith, who seems as morally rigid as he is incapable of abstract thought. Ed Kemper is a talker, but Jerry Brudos and Speck begin stonewalling Bill and Holden the moment they start reading from their prepared remarks. They make a sport out of toying with these stuffed shirts and they both require a little trickery on Holden’s part to get them talking, whether it’s offering Brudos a pair of high heels or encouraging the vicious misogyny that motivates Speck. What’s more, Holden sees these interviews as producing useful, actionable intelligence that can be applied in the field right away, before they get folded into some study that will take years to produce. In this episode alone, they’re able to draw up a profile on a rapist and murderer in Georgia that might not have been possible in the past.

But it’s Bill who’s perhaps the most interesting figure in this debate, because he’s caught in an uncomfortable place between both worlds. When Holden starts talking to Speck in his language (e.g. “What gave you the right to take eight ripe cunts out of the world?”), Bill is plainly appalled, and he remains livid as Holden makes more disturbing statements on the way to the car. (“When he talked about feeding [the birdie] meatloaf through an eyedropper, I actually felt sorry for the guy.”) He largely agrees with Carr’s point of view, saying that “we can learn from snails,” but he knows what it’s like to pry information from a subject on the ground. Sticking to the script isn’t always feasible.

The solution Bill suggests gets them into trouble, and reflects how dangerous his neither-here-nor-there position is. Leaving Holden’s filthy role-play off the transcript is not something Holden himself had ever considered. He’s confident that he’s following his instincts and getting excellent material as a result. Bill can anticipate the poor reaction the interview will get from Carr, so his instinct is to protect his partner and clean up the transcript, which of course gets Smith involved, along with everyone else once the Office of Professional Responsibility comes poking around on Speck’s behalf. Carr wants to be transparent about their withholding of evidence. Bill wants to close rank, arguing that it would bring unnecessary trouble down on them. Bill wins, but Smith appears willing to play the whistle-blower in the end.

“Episode 9” brings David Fincher back in the director’s chair for a riveting hour on ethics and methodology, which deepens the show beyond what might have been expected of it. Because Mindhunter is based on a book co-written by John E. Douglas, who doubles as the inspiration for Holden, there was reason to suspect that it would be the origin story of the FBI’s serial crime unit, emphasizing how important these developments were for understanding the minds of certain violent criminals. Yet the focus hasn’t been entirely on profiling serial killers and applying that information on real cases. It’s also circled back to the profilers themselves, particularly Holden, and questioned the serious missteps that they’re making on the job. Speck puts perhaps too fine a point on it during the interview — “You’re crazy, there’s a fine line separating you from me” — but the show gains in complexity by implicating the agents’ behavior in the field. They’re not perfect.

In that respect, Mindhunter aligns with other Fincher serial-killer thrillers, especially Zodiac, which gets the killings out of the way early on and spends the rest of the time following obsessives down the rabbit hole. Fincher knows the type well, the “how do we get ahead of crazy if we don’t know how crazy thinks” mentality. It’s an unflattering look for someone who purports to be an expert and professional to stoop to Speck’s level and reflect his language back to him. And the willingness of Mindhunter to allow its heroes to look bad is a big part of what makes it work.

Dillingers

• After a one-episode break, we’re back to cold opens involving the BTK killer, who’s dressed for the part. The exterior shot of the house, with Rader behind the blinds, is haunting in such a distinctly Fincher way. There’s a gray-black menace to the place, as if there’s a ghoul inside.

• The pattern of interviews and real cases sometimes syncs up too neatly. We hear Speck talk about how an intended robbery led to rape, which led to murder, which led to more murders. Then we cut later to Georgia, where an intended rape perhaps leads to inadvertent murder. One revelation applying so neatly to another strengthens Holden’s argument, but it’s starting to feel dramatically convenient.

• The whipping of the bird into the fan is the biggest shock the show has produced so far. When you’re incapable of empathy, it’s possible to dispatch of life this casually.

• Tremendous scene involving Principal Wade’s wife paying Holden a visit to shun him for supporting her husband’s termination. Fincher’s staging of it raises the tension, with the two on opposite ends of the hallway and the elevator opening and closing persistently. Neither of them want to be near each other.

• My friend Alyssa Rosenberg, a columnist for the Washington Post, has a good essay up about Mindhunter as a show about misogyny and why it’s the perfect show for a moment when contempt and violence toward women has surfaced in the news. Read it here.